its not really a sustainer pedal, its just a word they put on there to add placebo effet to the small amount of percieved sustain increase you get with compression.
its a compression pedal, pure and simple. do you understand how compression works? im assuming you dont by your post. that goes for all that have posted in this thread so far.
go to wiki, type in compression and go to audio compression. you want audio dynamic compression, not audio data compression (thats mp3 type stuff).
it's a very very useful tool, more so for recording than playing live, and more so after the event (as in mixing) than during the actual recording (aka tracking).
basically, these guitar boxes are a bit of a gimmick, and arent as good as their rackmount counterparts, but they still have their uses and may suit the sound youre after.
they are not sustainer pedals in the sense you suspected, but compression can create percieved sustain, by virtue of decreased dynamic range.
